It was -3° this morning but the temperature has gone up 13° over the course of the day. What is the temperature now?

The temperature now is 10°. (-3° + 13° = 10°)

To find the current temperature, add the increase in temperature to the morning temperature.

Morning temperature: -3°
Increase in temperature: +13°

Calculating: -3° + 13° = 10°

Therefore, the current temperature is 10°.